Method for selectively removing chromates

ABSTRACT

Removal of chromate ions from water containing a large amount of total dissolved solids by contacting the water with an acidified weakly basic anion exchange resin, preferably in .[.microporous.]. .Iadd.macroporous .Iaddend.form, to selectively remove chromates from the water.

BACKGROUND OF THE INVENTION

Inorganic chromates are frequently employed as corrosion inhibitingmaterials in industrial waters, particularly in circulating watersystems for cooling towers. The chromates are sometimes used alone butfrequently are employed in conjunction with molecularly dehydratedphosphates, the combination functioning as scale and corrosioninhibitors to protect metal conduits and other metallic surfaces withwhich the circulating water comes in contact.

Water being used as a cooling medium in industrial applicationsfrequently is very high in dissolved solids, usually exceeding 1000parts per million or so. When a cooling tower which has been treatedwith chromate corrosion inhibitors reaches a certain concentration levelof dissolved solids, it is customary to blow it down by discharging aportion of the water and adding fresh makeup water having a lowerdissolved solids content. A typical cooling water blow-down mightcontain, for example, 20 parts per million of a chromate and 1200 partsper million of other dissolved salts such as sodium chloride, calciumsulfate, magnesium nitrate, and the like.

When such waters are discharged into natural water courses or to sewagesystems, the high chromate concentration may pose a problem because oflocal regulations covering the nature of effluents which can bedischarged. The presence of the large dissolved solids content, however,makes it quite difficult to remove the chromate ions selectively whileleaving the relatively innocuous salts for discharge.

DESCRIPTION OF THE PRIOR ART

Some attempts have been made to use ion exchange resins in the removalof chromate ions from waste waters containing high levels of totaldissolved solids but not with complete success. In some cases, theresins have a distinct fouling tendency and some are not stable tooxidation. Furthermore, strongly basic anion exchange materials whenused for this purpose do not have particularly high loading capacitiesnor high regeneration efficiencies.

SUMMARY OF THE INVENTION

The present invention makes use of weakly basic anion exchange resinsunder certain operating conditions for a selective removal of chromateions from waters having high total dissolved solid contents.Specifically, it has been found that macroporous weakly basic anionexchange resins which have been at least partially acidified prior touse can selectively remove chromate ions from industrial waters havinglarge amounts of total dissolved solids. The weakly basic resins .[.havethe advantage of providing higher loading capacities,.]. are lesssubject to organic fouling, and have a high regeneration efficiency. Thebest results are obtained in the process of the present invention whenthe pH of the water being treated is adjusted to a value of from 3.0 to5.0 before contacting the same with the resin, and preferably to a valueof from 3.0 to 3.5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The weak base anion exchange resins which provide the best results inthe practice of the present invention are those which are macroporous,having pore diameters ranging from about 5 to 100 Angstroms or more.Chemically, these materials can be of different constitution, but shouldcontain an intermediate amine as the exchange site. Particularly goodresults are obtained when the resin used is a mixture of secondary andtertiary amines, since these resins are considerably more stable tooxidation than are resins which have reaction sites consisting solely ofprimary and secondary amines.

The best resins found for this purpose are the polystyrene divinylbenzene amines which are prepared by nitration and subsequent reductionof styrene-divinyl benzene copolymers. Other materials which can be usedin accordance with the present invention are the weak base anionexchange resins such as the amine derivatives of epichlorohydrin andpolyvinylpyrrolidone.

Prior to contacting the water to be treated with the resin, the resin isacidified by treatment with a dilute mineral acid such as sulfuric acidor hydrochloric acid to at least partially convert the amine to its acidform. It is not always necessary that the amine be treated withstoichiometric amounts of acid sufficient to cause complete conversionto the acid form, but satisfactory results can be obtained using anamount of acid ranging from about 10% to 100% of the stoichiometricamount.

The treatment can be effected in the usual way by passing the waterthrough a bed of particles of the weakly basic anion exchange resin.

To illustrate the beneficial effect of acidification of the resin, atest was made using 140 ml. of a weak base ion exchange resin (IRA- 93,a polystyrene-divinyl benzene amine supplied by Rohm and Haas) in aone-half inch internal diameter column with a forty-two inch bed heightat flow rates up to .[.14,000 gallons per cubic foot.]. .Iadd.2 gallonsper minute per cubic foot.Iaddend.. The following table illustrates theresults obtained:

                  TABLE I                                                         ______________________________________                                                                             Average                                                                       CrO.sub.4                                                  Flow rate, Capacity,                                                                             leakage,                                 Ionic form                                                                            Influent pH                                                                             g.p.m./cu.ft.                                                                            lbs./cu.ft.                                                                           p.p.m.                                   ______________________________________                                        H.sub.2 SO.sub.4                                                                      3.5       2.0        5.9     <0.1                                     H.sub.2 SO.sub.4                                                                      3.5       1.0        6.2     <0.1                                     H.sub.2 SO.sub.4                                                                      4.5       1.0        4.9     <0.1                                     Free amine                                                                            3.5       1.0        5.6     10                                       ______________________________________                                    

The water employed in the above test contained 1400 parts per million oftotal dissolved solids (calcium, magnesium, sodium, sulfate andchloride) plus 49 parts per million of chromate ion. The temperature ofthe influent was held between 75° and 85° F.

The resins can be regenerated by treatment with an alkali such as sodiumhydroxide, potassium hydroxide, or ammonium hydroxide to remove thechromate ions.

After exhaustion, the column with a loading of 4.9 pounds chromate percubic foot was regenerated with 560 ml. of 4% sodium hydroxide,amounting to a concentration of 10.3 pounds of sodium hydroxide percubic foot of resin. This large excess of sodium hydroxide was used toassure a complete regeneration. Samples were taken periodically duringthe one hour contact of the sodium hydroxide with the resin. The totalchromate .Iadd.loaded on the column was .Iaddend.eluted by theregenerant .[.was 5.14 pounds per cubic foot.].. Of this, 96% was elutedby the first 420 ml. of regenerant.

As previously indicated, the best results are obtained using an influentpH of 3.0 to 5.0. However, the resins will remove chromate ions at lowerpH values than 3.0 but the stability of the resin is significantlydecreased at these pH values. The highest resin loading occurs at aninfluent pH of 3.5.

The reactions involved are believed to be represented by the followingequations: ##STR1##

It should be evident that various modifications can be made to thedescribed embodiments without departing from the scope of the presentinvention.

I claim as my invention:
 1. The method of selectively removing chromateions from water containing .[.a large amount.]. .Iadd.at least 1,000parts per million .Iaddend.of total dissolved solids which comprises.Iadd.adjusting the pH of the water being treated to a value from about3.0 to about 5.0, .Iaddend.acidifying a weakly basic anion exchangeresin .Iadd.in the macroporous form .Iaddend.and thereafter contactingsaid water with the acidified resin to cause selective removal ofchromate ions from said water. .[.2. The method of claim 1 in which saidanion exchange resin is in macroporous form..]. .[.3. The method ofclaim 2 in which said resin has pore diameters in the range from about10 to 100 Angstroms..]. .[.4. The method of claim 1 in which the pH ofthe water being treated is adjusted to a value of from 3.0 to 5.0 beforecontacting the same with said resin..]. .[.5. The method of claim 1 inwhich the pH of the water being treated is adjusted to a value of from3.0 to 3.5..].
 6. The method of claim 1 in which the acidification ofthe resin consists in treating said resin with sulfuric acid.
 7. Themethod of claim 1 in which the acidification of the resin consists intreating said resin with hydrochloric acid.
 8. The method of claim 1 inwhich said resin is a polystyrene-divinyl benzene amine resin. .Iadd.9.A method for selectively removing chromate ions from circulating watersystems which have been treated with chromate corrosion inhibitors,which comprises:discharging a portion of the water from the system whena level of at least 1,000 parts per million total dissolved solids ispresent, to obtain a blow-down and adjusting the pH of said blow-down toa value of about 3.0 to about 5.0; acidifying a macroporous weakly basicanion exchange resin; and thereafter contacting the blow-down with saidresin to selectively remove the chromate from the blow-down to obtain ablow-down substantially free from chromate ions. .Iaddend. .Iadd.10. Themethod of claim 9 wherein the pH of the blow-down is adjusted from about3.0 to about 3.5. .Iaddend. .Iadd.11. The method of claim 9 wherein saidresin is treated with a dilute mineral acid to acidify it prior tocontact with the blow-down. .Iaddend. .Iadd.
 12. The method of claim 11wherein the dilute mineral acid is sulfuric acid. .Iaddend..Iadd. 13.The method of claim 11 wherein the dilute mineral acid is hydrochloricacid. .Iaddend. .Iadd.14. The method of claim 11 further includingtreating the resin, after contact with the blow-down, with an alkali toregenerate the resin by removing the chromate ions. .Iaddend. .Iadd.15.The method of claim 11 in which said resin is a polystyrene-divinylbenzene amine resin. .Iaddend.